Backlight and display utilising same

ABSTRACT

A backlight including a light guide ( 10 ) wherein light from one or more light sources ( 32 ) is constrained by total internal reflection and from which light may be output to provide backlighting for a display screen, and a light reservoir ( 34 ) for feeding said light guide with light. The light guide has an input surface ( 20 ) including incoupling elements ( 24 ) rising therefrom and extending into the light reservoir, the incoupling elements having at least three mutually non-coplanar sidewall sections ( 26 ) extending transversely from the input surface, each sidewall section covering a different part of the input surface. The light reservoir is arranged such that light passing through one area of the light reservoir is capable of entering the light guide through each of said sidewall sections.

This invention relates to backlights, and it relates especially, thoughnot exclusively, to backlights intended for use with slim-line visualdisplay devices, and to provide thereto light of good uniformity andsufficient brightness.

Devices associated with such backlights are typically capable of use intelevision displays, monitors and the like. Whatever the proposed use ofthe display, however, the invention also encompasses displaysincorporating, or used in conjunction with, such backlights.

The invention provides special benefits when used in conjunction with,or as part of, a so-called foil display, in which a mobile foil,disposed between a viewing screen and a backlight, is locally attracted,by the application of dynamic voltage waveforms, into contact with theoutput surface of the backlight at positions where (depending on thepicture content to be displayed) light is to be emitted from thedisplay. The local points of contact between the foil and the backlightsurface represent bright pixels and the points where no such contact ismade are correspondingly dark, and examples of such devices and lightingarrangements therefore can be found for example in WO-A-99/28890;WO-A-00/38163; WO-A-00/50949 and WO-A-01/63588.

A difficulty with slim-line visual displays, including theaforementioned foil displays, arises in coupling enough light into thebacklight to illuminate the display with sufficient brightness anduniformity. Typically, the backlight comprises a thin, rectangularplate-like light guide, juxtaposed with the viewed screen of thedisplay, into which light is edge-coupled. The light so coupled into thelight guide is in general constrained therein by total internalreflection (TIR), and coupled out only at the locations of pixels whichare to appear bright in accordance with the image to be viewed by way ofthe screen. With such arrangements, light tends to be lost at variouslocations, but the principal difficulty arises in edge-couplingsufficient light into the light guide. In this respect, the number oflamps that can be used for illumination of the display is limited by thespace available at the edge of the light guide, placing a practicallimit upon the maximum brightness available.

Moreover, those pixels that are to appear bright extract light from thelight guide, thereby limiting the amount of brightness available forother pixels, and adversely affecting the uniformity of the display asregards illumination. This effect of course is dynamic, as it dependsupon the image content.

Both of the above effects (limit in overall illumination and dynamicnon-uniformity) are particularly noticeable with large displays.

Direct backlighting with diffusers, as may be used for example withliquid crystal displays (LCDs), is not viable with displays usingcontact light outcoupling, such as foil displays, which rely upon TIRfor good contrast.

The invention addresses the problems of brightness and uniformity inbacklights utilising TIR, and accordingly aims to provide improvedbacklights and thus improved displays incorporating them.

According to the invention from one aspect there is provided a backlightincluding a light guide wherein light from one or more light sources isconstrained by total internal reflection and from which light may beoutput to provide backlighting for a display screen, and a lightreservoir for feeding said light guide with light,

the light guide having a substantially planar construction and includingan output surface from which light may be output and an opposing inputsurface disposed to receive said light,

said input surface including incoupling means rising therefrom andextending into the light reservoir, the incoupling means having at leastthree mutually non-coplanar sidewall sections extending transverselyfrom said input surface, each sidewall section covering a different partof the input surface, the light reservoir being arranged such that lightpassing through one area of the light reservoir is capable of enteringthe light guide through each of said sidewall sections.

By this means it will be appreciated that an input arrangement isprovided in which input light is spatially mixed and effectivelyintroduced into the light guide so as to ensure TIR propagation from oneor more lamps which are not constrained by the need for positioning ator adjacent to an edge of the light guide.

Attempts have previously been made to inject light transversely into alight guide by incorporating light sources into channels in the lightguide surface. Such an arrangement is disclosed in German patentapplication No. 101 02 587.4. In such arrangements, however, the channeldimensions are dictated by the size of the lamps and the light guides soproduced tend to be large and heavy. Furthermore, such arrangementsexhibit other characteristics which can reduce their suitability for usein displays utilising contact light outcoupling.

The present invention, on the other hand, opens up the opportunity ofproviding a plurality of lamps in a light-box closely coupled to theinput surface of the light guide, with sufficient incoupling elementsrising from said input surface to permit the injection of significantamounts of light into the light guide at angles consistent with thepromotion of TIR.

In preferred embodiments of the present invention, the incoupling meansare formed as an array of incoupling elements rising from the inputsurface of the light guide. In preferred embodiments, the array maycomprise a plurality of substantially parallel, spaced apart, elongateridge-like members running transversely across the input surface of thelight guide. The ridge-like members are preferably substantiallyrectangular in cross-section and are thus formed with substantially flatouter surfaces and upright sidewalls that are substantially orthogonalto the input surface of the light guide. Alternatively, the incouplingelements may comprise an array of cubes or cylinders, each formed with arespective, substantially flat outer surface and respective uprightsidewalls substantially orthogonal to the input surface of the lightguide.

Preferably, in any event, the outer surfaces of the incoupling means arerendered substantially absorbent with respect to stray light incidentthereon from within the light guide, in as far as this stray light isnot constrained inside the light guide by TIR, but substantiallyreflective with respect to light otherwise incident thereon. This ispreferably achieved by depositing, upon each of said outer surfaces, alight-absorbent layer facing the light guide, the light-absorbent layerbeing substantially not in optical contact with the incoupling elements,followed by a white diffusively reflective coating facing the lightreservoir.

In preferred embodiments, reflective means are disposed in spacesbetween the incoupling means. These may include portions inclined to thesidewalls of said incoupling elements. The intermediate reflective meanspreferably comprise laminar material configured into a substantiallyinverted V-shape; said laminar material extending upwardly fromlocations adjacent to the junctions of the sidewalls of adjacentincoupling elements with said input surface, but having substantially nooptical contact with said input surface. This requirement for lack ofoptical contact between the reflective means and the input surface ofthe light guide arises because of the desire not to outcouple usefullight that is propagating under TIR conditions in the light guide. Inthis connection, it is preferred that air be disposed between thereflective means and those regions of the input surface of the lightguide which that it overlies.

Preferably, surfaces of the intermediate reflective means that face thelight reservoir are rendered diffusely reflective of light and surfacesthereof which face towards the light guide are rendered light absorbent.Where the reflective means is a laminar material, respective reflectiveand absorbent coatings may be applied to the appropriate surfacesthereof.

Preferably the light source or light sources comprise tubularfluorescent lamps disposed with their axes substantially parallel tosaid input and output surfaces of the light guide. Moreover, where theincoupling elements comprise elongate ridges, the axes of said lamps arepreferably disposed parallel to said ridges. In alternative embodiments,the lamps may comprise light emitting diodes (LEDs), or a mixture ofvarious light sources could be used.

A significant advantage of the invention over prior art arrangements isto permit the number and arrangement of light sources to be selectedindependently of other parameters of the system, thus permitting theoverall light input to readily be scaled to meet operationalrequirements.

The light source or light sources are preferably mounted in a light boxwith reflective surfaces, preferably white diffusely reflectivesurfaces, to maximise the amount of light available for injection intothe light guide.

In order that the invention may be clearly understood and readilycarried into effect, one embodiment thereof will now be described, byway of example only, with reference to the accompanying drawings, ofwhich:

FIG. 1 shows schematically and in cross-sectional view, a part of adisplay in accordance with one example of the invention; and

FIG. 2 shows in schematic and simplified form a display in accordancewith one example of the invention utilising the backlight shown in FIG.1.

Referring now to FIG. 1, a display is shown including a backlightcomprising a light guide 10 and a lightbox 34, and a display screen 44.The display screen 44, to be described in further detail below, includesa matrix of electrodes whereby transmitted light intensities onpixel-sized areas are modulated for generating image frames by means ofimage scanning circuitry (not shown) in accordance with a received imagesignal.

Light is intended to propagate within the light guide 10 by TIR, asindicated schematically by the arrowed lines 12 and 14. The light guide10 is in this embodiment thin and plate-like in construction, formedfrom a solid transparent material such as glass or a plastic material.The light guide 10 has edges 16 of relatively small dimensions, whileits upper and lower surfaces 20 and 22 respectively, as seen in thedrawing, are generally rectangular and of relatively large dimensions,commensurate with a display area. The edges 16 of the light guide areprovided with a specular reflective coating 18 or a diffuse reflectivecoating 18, the diffuse reflective coating not being in optical contactwith the surface of the edge 16.

The lower surface 22 of the light guide 10, as viewed in the drawing, issubstantially planar and constitutes in this example the output surfaceof the light guide.

The upper surface 20, as viewed in the drawing, constitutes the lightinput surface and includes a one-dimensional array of rising incouplingelements 24 spaced apart. In this example, each incoupling element 24comprises an elongated ridge of substantially rectangular cross-sectionformed as part of, and disposed transversely across, the surface 20. Theelements 24 each have sidewalls 26 and an outer surface 28; thesidewalls being, in this embodiment, upright and substantiallyorthogonal to the input surface 20 of the light guide 10 though thisneed not necessarily be the case.

In the spaces between the incoupling elements 24 are provided reflectiveelements 30, which inhibit light entering the light guide through theinput surface in the spaces, at angles which would not promote TIR inthe light guide 10. The reflective elements 30 in this example compriselaminar members formed into inverted V-shapes, effectively extendingfrom the base of the sidewall 26 of one incoupling element 24 to thebase of the sidewall 26 of an adjacent incoupling element 24. Thereflective elements 30 are arranged such that there is substantially nooptical coupling between the reflective elements 30 and the inputsurface 20 of the light guide 10 in order to reduce unwanted outcouplingof light propagated by TIR in the light guide 10.

An array of light sources such as tubular fluorescent lamps 32 isdisposed in a lightbox 34 and closely coupled to the input surface 20 ofthe light guide 10.

The incoupling elements 24 may be configured in various differentformats and, instead of being formed as a one-dimensional array ofelongated ridges of rectangular profile, may alternatively be formed,for example, as a two-dimensional array of upstanding post-like elementssuch as cubes or cylinders. In any event, however, the sidewalls 26 areconfigured to provide the sole access for light from the lightbox 34into the light guide 10, and vice-versa. The endwalls 28 of theincoupling elements are first provided with a light-absorbing coatinglayer such as a black layer 36 that substantially does not opticallycouple with the incoupling elements 24 and on top of that is providedwith a reflective coating 38 which is preferably a white diffusereflective coating. The purpose of the coating 36 is to absorb any straylight, incident from within the light guide 10 and not constrainedwithin the light guide by means of TIR, that strikes the outer surface28 since such light, if it were allowed to continue by a back-reflectionfrom the outer surface 28, would not be subject to TIR and thus mightemerge as unwanted stray light from an endwall 22. The reflectivecoating 38 inhibits light incident from the light box 34 from enteringthe light guide at angles which would not promote TIR in the lightguide, and redirects such incident light back into the light box 34.

Likewise, the reflective elements 30 are provided with a reflectivecoating, preferably a white diffuse reflective coating, on the surfacethat faces the lamps and with a black absorbent coating on its surfacethat faces the light guide 10. Moreover, air is disposed in the area 40between the reflective elements and the light guide to avoid opticalcoupling between the reflective elements and the light guide surface 20and thus the outcoupling of useful light that is propagating in thelight guide under TIR conditions.

The lightbox 34 is coated with diffusive white reflective material onits internal surface 42, so as to maximize the amount of light from thelamps that is constrained within, and thus usable by the system.

Instead of, or in addition to, the array of tubular fluorescent lamps32, other light sources such as LEDs may be used within the lightbox 34.

Where tubular fluorescent lamps are used, their axes preferably runparallel to the surface 20 and to the long axes of the incouplingelements 24, where these are elongated in form. There is no directcorrelation between the number of light sources used and either thenumber of incoupling elements provided or the dimensions of the lightguide; the various values and dimensions can be arranged to suit theoperating requirements of the system as a whole.

As shown in FIG. 1, the light reservoir formed by the lightbox 34extends further from the input surface 20 of the light guide than theincoupling elements 24. This allows spatial mixing in the light box sothat light passing through one area, such as area A shown in a dottedcircle in FIG. 1, is capable of entering the light guide through each ofthe incoupling elements 24, thus providing homogenization of the inputlight. For illustrative purposes, in FIG. 1, light from the area Awithin the light reservoir formed by the lightbox 34 is shown asentering the light guide 10 via three different incoupling elements 24.

There is sufficient space in the lightbox, and the endwalls aresufficiently small in height, for the light sources 32 to be spacedremote from the endwalls 28 in a direction away from the plane of thelight guide. As shown in FIG. 1, the endwalls extend into the lightbox34 to an extent defined by plane 46 shown as a dotted line. The lightsources are placed further from the light guide 10 than the plane 46,thus allowing the light sources to be arranged in a desiredconfiguration independently of the arrangement of the incouplingelements 34. Conversely, the incoupling elements 34 may also be arrangedin a desired configuration independently of the arrangement of the lightsources 32. Preferably, the number of incoupling elements 34 is greaterthan the number of light sources, to provide increased degree ofhomogenization for a given number of light sources. In any case, thepattern in which the light sources are arranged in a direction parallelto the plane of the light guide does not need to correspond as regardspitch with the pattern in which the incoupling elements are arranged inthe same direction.

The reflective elements 30 need not provide tilted reflective surfacesas shown in the drawing and can take any convenient and practical form,bearing in mind their intended use to divert light into the light guide10 through the sidewalls 26 of the incoupling members 24 and therequirement that substantially no optical coupling should exist betweenthe reflective elements 30 and the input surface 20 of the light guide.The sidewalls are preferably substantially orthogonal to the inputsurface 20 though, as previously mentioned, this is not essential andthey may be disposed at other orientations.

FIG. 2 shows in further detail and schematically a foil display screen,associated with a backlight as illustrated in FIG. 1. For simplicity ofillustration, the light input arrangement for the light guide 10 isomitted in FIG. 2. It should be appreciated that the light sources 34are continuously activated during the operation of the display screen.The display screen is of the kind described in international patentapplications WO-A-99/28890; WO-A-00/38163; WO-A-00/50949 andWO-A-01/63588, the contents of which relating to the display screen areincorporated herein by reference.

The display screen 44 comprises a flexible member 54, typically alight-scattering polymeric foil, disposed between the output surface 56of the backlight 52 and a transparent plate 58. Electrode systems 60 and62 are arranged, respectively, on the output surface 56 and the innersurface 64 of the transparent plate 58. By locally generating apotential difference between the electrodes 60, 62, and the foil 54, byapplying voltages to the electrodes and the foil, forces are locallyexerted on the foil at each potential contact point (corresponding to apixel of the display) sufficient to either press the foil against theoutput surface 56 or keep it away from it, depending on whether, inaccordance with the content of an image to be displayed, light is or isnot to be coupled out of the backlight to emerge from the display atthat point. It will be appreciated that the applied voltages are used toscan the contact point relative to the plates in a two-dimensionalpattern, such as a conventional television raster.

The display screen 44 also has a covering screen element 66 which isconfigured so as to form an airtight connection with the backlight 52,whereby the space 68 can be evacuated.

While the present invention has particular applicability to the foildisplay described herein, it should be appreciated that the backlightdescribed herein can be used with any kind of display for which it maybe suited.

The invention encompasses display apparatus, such as broadcast/cable TVreceivers, specialist monitoring equipment, for medical, technical orforensic purposes for example, monitors for personal computers, anddisplays for portable electronic devices such as mobile telephones andpersonal digital assistants.

It is to be understood that any feature described in relation to any oneembodiment may be used alone, or in combination with other featuresdescribed, and may also be used in combination with one or more featuresof any other of the embodiments, or any combination of any other of theembodiments. Furthermore, equivalents and modifications not describedabove may also be used without departing from the scope of theinvention, which is defined in the accompanying claims.

1. A backlight including a light guide wherein light from one or morelight sources is constrained by total internal reflection and from whichlight may be output to provide backlighting for a display screen, and alight reservoir for feeding said light guide with light, the light guidehaving a substantially planar construction and including an outputsurface from which light may be output and an opposing input surfacedisposed to receive said light, said input surface including incouplingmeans rising therefrom and extending into the light reservoir, theincoupling means having at least three mutually non-coplanar sidewallsections extending transversely from said input surface, each sidewallsection covering a different part of the input surface, the lightreservoir being arranged such that light passing through one area of thelight reservoir is capable of entering the light guide through each ofsaid sidewall sections.
 2. A backlight according to claim 1, furtherincluding intermediate reflective means disposed in spaces between saidsidewall sections and configured to reflect light impinging thereon fromsaid light reservoir away from parts of the input surface in saidspaces.
 3. A backlight according to claim 2, wherein surfaces of saidintermediate reflective means which face away from the light guide arerendered diffusely reflective to light.
 4. A backlight according toclaim 2 or 3, wherein surfaces of said intermediate reflective meanswhich face the light guide are rendered light absorbent.
 5. A backlightaccording to claim 2, 3 or 4, wherein a gas is provided between theintermediate reflective means and those regions of the input surface ofthe light guide which it overlies.
 6. A backlight according to anypreceding claim, wherein the incoupling means further include endwallsections spanning the respective sidewall sections, said endwallsections being provided with reflective means configured to reflectlight impinging thereon from said light reservoir away from the endwallsections.
 7. A backlight according to claim 6, wherein surfaces of saidendwall reflective means that face away from the light guide arerendered diffusely reflective to light.
 8. A backlight according toclaim 6 or 7, wherein surfaces of said endwall reflective means whichface the light guide are rendered light absorbent.
 9. A backlightaccording to any preceding claim, wherein said one or more light sourcesare located in said light reservoir.
 10. A backlight according to anypreceding claim, wherein said one or more light sources are spacedremote from said incoupling means in a direction away from the plane ofthe light guide.
 11. A backlight according to claim 10, wherein thepattern in which the light sources are arranged in a direction parallelto the plane of the light guide does not correspond as regards pitchwith the pattern in which the incoupling elements are arranged in thesame direction.
 12. A backlight according to claim 11, wherein theincoupling means comprise a plurality of incoupling elements arrangedover the input surface.
 13. A backlight according to claim 12, whereinsaid incoupling elements comprise a plurality of elongate ridge-likemembers transversely spaced in one direction across the input surface ofthe light guide.
 14. A backlight according to claim 13, wherein theridge-like members are substantially rectangular in cross-section,having substantially flat endwalls and upright sidewalls that aresubstantially orthogonal to the plane of the light guide.
 15. Abacklight according to claim 12, wherein the incoupling elementscomprise a plurality of members spaced in two orthogonal directionsacross the input surface of the light guide.
 16. A backlight accordingto any of claims 12 to 15, wherein the height of each said incouplingelement, measured perpendicular to the plane of the light guide, is atleast as great as a width thereof measured parallel to the plane of thelight guide.
 17. A backlight according to any preceding claim, whereinthe one or more light sources comprise one or more tubular fluorescentlamps disposed with their axes substantially parallel to said input andoutput surfaces of the light guide.
 18. A backlight according to claim17, as dependent on claim 13 or claim 14, wherein the axis or axes ofsaid one ore more lamps are disposed parallel to said ridges.
 19. Abacklight according to any of claims 1 to 16, wherein the one or morelight sources comprise one or more light emitting diodes.
 20. Abacklight according to any preceding claim, wherein a mixture of varioustypes of light sources is used.
 21. A backlight according to anypreceding claim, wherein the light reservoir is formed in a light boxwith white diffusely reflective inner surfaces.
 22. A display for imagedisplay, including a backlight according to any preceding claim.
 23. Adisplay according to claim 22, the display comprising a screenconfigured to display images; the display screen comprising a mobileelement disposed adjacent to said backlight, and means utilizing dynamicvoltage waveforms for locally attracting said mobile element intocontact with the output surface of the backlight at positions where,depending on the picture content to be displayed, light is to be emittedfrom the display.